It is known in the glass manufacturing art that glass batch materials may be formed into agglomerates and that these agglomerates may then be discharged to a melting furnace for vitrification of the batch ingredients. These agglomerates are, generally, composite, integral, self supporting masses of batch materials and may be in the form of extrusions, discs, briquettes, or pellets. Exemplary teachings as to this may be found in U.S. Pat. No. 3,880,639. For further exemplification, as to a manner in which glass batch may be formed into agglomerates, as for example, by forming the batch into pellets with water, reference may be had to U.S. Pat. No. 3,914,364 which is assigned to the Dravo Corporation. U.S. Pat. No. 3,366,473 also discloses forming glass batch into agglomerates.
In the fibrous glass manufacturing industry, B.sub.2 O.sub.3 containing glasses have been manufactured extensively for some period of time. Exemplary of such glasses are those set forth in U.S. Pat. Nos. 2,877,124 and 2,882,173 which compositions are generally employed for thermal insulation products. Generally the glasses which have been commercialized in the past for such products may be referrred to as soda-lime aluminoborosilicate glasses in that they substantially comprise a combination of Na.sub.2 O, CaO, Al.sub.2 O.sub.3, B.sub.2 O.sub.3, and SiO.sub.2. The source of B.sub.2 O.sub.3 in these glasses has commonly been a borax, i.e., borax itself, anhydrous borax or 5 mole borax. If desired, boric acid may also be employed, as the B.sub.2 O.sub.3 source, as may ulexite or colemanite as taught in U.S. Pat. No. 3,274,006.
The manufacture of B.sub.2 O.sub.3 containing glasses, for example the above fibrous glass products, by the use of agglomerates is highly desirable. This is especially true in those instances where the agglomerates are heated at a temperature and for a time sufficient to remove water, i.e., dry them (as where the batch has been formed into pellets with water) and then to further heat these pellets to an elevated temperature, which is short of their sintering or melting temperature, followed by discharging such preheated pellets into the melting furnace. Applicant has found however, that all B.sub.2 O.sub.3 sources are not equivalent for such a process. For example, when a borax is employed as the source of B.sub.2 O.sub.3, agglomerates formed by consolidating the batch into individualized units with water, for example as pellets, may slump when heated which substantially precludes the pellets from being conveyed to the melting furnace in an acceptable economical manner. That is, when a borax is employed to form batch into pellets with water, for example pellets on the size of at least about 1/4 inch, and generally in the range of about 1/4 to 3/4 inch, with the water employed for pelletization being about 5 to about 20 percent by weight, these pellets have a latent, unacceptable rheological property. As several layers of these pellets are first heated under conditions of time and temperature to dry them and then further heated (preheated) to a temperature short of their sintering, or fusion, temperature, instead of remaining as non-aggregated, discrete flowable pellets which can be conveniently conveyed to a melting furnace, they transform, or coalesce into an unacceptable, aggregated mass. Boric acid likewise, is not suitable for such a process because it has an unacceptably low softening or fusion point which severely handicaps the ability to heat such pellets, prior to discharging or conveying them into the melting furnace, to an elevated temperature. Colemanite and calcined colemanite are also not satisfactory for purposes of making the above indicated B.sub.2 O.sub.3 containing glass products from preheated pellets. For example, calcined colemanite is economically not suited for soda-lime alumino borosilicate glasses because these glasses are relatively inexpensive glasses and the employment of such a material as the B.sub.2 O.sub.3 source in the agglomerate cannot be competively tolerated.
U.S. Pat. Nos. 4,074,989, 4,074,990, and 4,074,991 are directed to methods for preparing B.sub.2 O.sub.3 containing batch in the form of pellets respectively employing anhydrous boric acid, colemanite and boric acid. As indicated previously, such materials are not suitable for the purposes contemplated herein. For example, B.sub.2 O.sub.3 has a melting point on the order of 450.degree. to 500.degree. C. which substantially precludes pellets made with such a material from being heated to an elevated temperature. Boric acid, on the other hand, tends to steam distill when the water containing agglomerates are heated and thus cause B.sub.2 O.sub.3 losses.
U.S. Pat. No. 4,074,990 discloses the use of colemanite as a batch ingredient for glass manufacturing. If the colemanite is not calcined, in accordance with that patent, the agglomerates (pellets) cannot be heated above 410.degree. C.; otherwise the pellets will disintegrate. Obviously this is not acceptable because it limits the temperature to which the pellets can be heated before being supplied to a melter and this is an especially serious handicap when it is desired to salvage sensible heat in flue gases by using them to preheat the agglomerates by passing the gases directly through a bed of such agglomerates. If it is desired to heat the pellets above 410.degree. C., this patent states that the chemically bound water must be removed, by calcination, prior to using the material to form pellets. Such required pre-processing serves to increase batch cost and, consequently, glass cost.
Thus, in accordance with this invention, Applicant provides for an improvement in processes of the above type as relates to glass manufacturing wherein a B.sub.2 O.sub.3 --containing glass is produced by forming batch ingredients therefor, including an uncalcined, mineral sodium calcium borate which contains about 5 or more moles of chemically combined water as a source of B.sub.2 O.sub.3, into a plurality of individual agglomerates with water. Preferably the agglomerates will be in the form of pellets containing about 5-20% by weight free water. These agglomerates may then be dried, under conditions of time and temperature to remove substantially all unbound water and convert them into dried, free-flowing agglomerates; the dried, free-flowing agglomerates may then be further heated, or preheated, to temperature higher than that needed to remove substantially all chemically bound water, for example a temperature in excess of about 500.degree. C. but short of the fusion temperature of the composition, to produce a mass of hot, free-flowing non-aggregated, dried agglomerates which may be transported in any convenient manner to a glass melter for melting. There is absolutely nothing in the above prior art which would suggest, certainly not with any predictability of success, that such borates can be employed as the source of B.sub.2 O.sub.3 in such agglomerates so as to allow one to practice such a process without substantial steam distillation losses and without disintegration by release of chemical water.
Acceptable results will be realized when the borate is the major (e.g. greater than 50%) source of B.sub.2 O.sub.3 in the batch. Outstanding results are obtained when such borate is present as substantially the sole source or, for example, an admixture with a borax may be employed with the weight ratio of such borate to such borax being at least about 2:1. The batch may be formed into agglomerates using conventional techniques. For example, when pellets are formed, these pellets may be formed by hand, but it is preferred to employ commercial pelletizing equipment such as a rotary disk like that commercially available from the Dravo Corporation. When pelletizing on a disk with water, it will be found that most preferably the particle size of the ulexite will be -200 mesh. Outstanding results will be obtained in practicing this method when producing a soda-lime alumino borosilicate glass consisting essentially of the following in approximate weight percent: SiO.sub.2 59-64; Al.sub.2 O.sub.3 3.5-5.5; CaO 7.5-9.5; MgO 0.1-3.7; B.sub.2 O.sub.3 4-12; Na.sub. 2 O 12-16; K.sub.2 O 0.2-2.8; Fe.sub.2 O.sub.3 0.1-0.3; TiO.sub.2 0-0.4; SrO 0-0.4; BaO 0-2.4; Li.sub.2 O 0-0.2; and SO.sub.3 0-0.5. Preferably the batch for such a composition will comprise sand, clay, dolomite or burnt dolomite, the uncalcined, mineral sodium calcium borate and soda-ash. When producing a Na.sub.2 O and B.sub.2 O.sub.3 containing glass, as will subsequently be seen, by using such borate and a sodium carbonate as the sources of B.sub.2 O.sub.3 and Na.sub.2 O, these materials uniquely interact to form agglomerates having unexpectedly high dry strengths. The significance of this is readily apparent.
Uncalcined, mineral sodium calcium borates contemplated for use herein may be specifically exemplified by the formula NaCaB.sub.5 O.sub.9.X H.sub.2 O, wherein X is between 5 and 8. More specifically such borates are represented by natural occurring deposits of ulexite and/or probertite. Such deposits will compositionally vary depending upon the source. For example, domestic source ulexite on a wet basis generally includes about 25 to 29 percent B.sub.2 O.sub.3, about 21 to 26 percent CaO, about 5 to 13 percent SiO.sub.2, about 4 to 5 percent Na.sub.2 O, and on the order of slightly less than 1 to slightly in excess of 2 percent by weight Al.sub.2 O.sub.3. Such ulexites include small amounts of other metal oxides and also volatiles such as water and carbon dioxide. Ulexite obtained from Turkey contains probertite and typically will contain about 38 to 39 percent B.sub.2 O.sub.3, 15 to 18 percent CaO, 2 to 3 percent SiO.sub.2, 4 to 7 percent Na.sub.2 O, and trace amount of other oxides including Al.sub.2 O.sub.3. That material likewise will include volatiles such as for example, water. Ulexite and probertite can be easily detected in these sources by x-ray analysis.